copied from another site in regards to holding your breath...
Of paramount importance when training with barbells is maintaining the spine's normal anatomical position. What is normal? When you stand up straight, your spine will assume something of an S-shape if viewed from the side. Near your shoulders, your spine traces the top of the S and demonstrates what is called kyphotic curvature. Your lower back also has a curve, or an arch, referred to as a lordotic curvature. This is normal anatomical position. For the purposes of this article, we are primarily interested in the lower back and keeping it in this extended, normal position when lifting weights.
Maintaining your lordotic curvature when under a load is function of trunk rigidity. The more stable and unmoving your torso, the more efficiently and safely you can transmit force to the load you wish to move. Your spinal erectors will provide isometric support for the spine in the back. The front, or anterior aspect, has only the abdominal musculature, with no corresponding bony structures to maintain stabilization. If you are to more effectively use those abdominal muscles, you need to pressurize your abdominal cavity using what is called the Valsalva maneuver. This is simply taking a big breath and holding it while closing the vocal folds of the throat, known as the glottis.
To perform the Valsalva effectively, you need to use your diaphragm to push your belly outward while inhaling. Just filling the lungs with air and raising the chest is less effective than relaxing the abdomen and pushing down with the diaphragm to stick your stomach out. Once your belly is properly protruding and you are holding that breath, you forcefully contract your abdominal muscles to create a maximally rigid torso. Now you are ready to perform your lift. When done correctly, this will markedly increase intra-abdominal pressure along with blood pressure and it will feel a little like your head wants to explode. By actively maintaining your lumbar curvature through the isometric contraction of the spinal erectors and the usage of the Valsalva maneuver, you can safely handle very heavy weights without fear of injuring your lower back.
A commonly voiced argument against holding the breath while lifting is the possibility of experiencing an aneurysm, or the bursting of a blood vessel in the brain or other part of the body. Such conditions are exceedingly rare and are primarily determined by genetic predisposition to such events. As Mark Rippetoe has rather humorously noted, "Parents cause aneurysms. Lifting weights does not." In healthy individuals, there is nothing to fear from performing the Valsalva maneuver. In fact, by not holding your breath during heavy attempts, you markedly increase your chances of spinal injury. Given that there are almost no recorded cases of anyone anywhere dropping dead from holding their breath while lifting, but there are innumerable instances of painful and debilitating back injuries from lifting improperly, you would be well advised to use the Valsalva on all of your strength-based lifts.